1. Field of the Invention
The present invention relates to a torque distribution control apparatus for a four-wheel drive vehicle.
2. Description of the Related Art
Japanese Patent No. 4082548 discloses an electronically controlled four-wheel drive vehicle in which the driving torque to be distributed from an engine to sub-driving wheels is set on the basis of a driving state of a vehicle obtained from fuel injection (FI) information (e.g., engine speed, suction negative pressure, suction air amount), automatic transmission (AT) information (e.g., shift position, torque ratio of torque converter), and antilock brake system (ABS) information (e.g., vehicle speed, brake signal).
When a state of a four-wheel drive vehicle including main driving wheels and sub-driving wheels is changed from a deceleration state in which an accelerator pedal is not pressed to an acceleration state in which the accelerator pedal is pressed, backlash of gears caused during deceleration in a power transmission system from the engine to the main driving wheels vanish at once during acceleration, and this causes rattling noise of the gears. Moreover, when the engine speed is lower than or equal to an idling speed, rattling noise of the gears is continuously caused by the changes in engine speed.
In this case, the driving torque of the engine is transmitted only to the main driving wheels when the accelerator pedal is not pressed. In contrast, when the accelerator pedal is pressed, the driving torque of the engine is also transmitted to the sub-driving wheels because of engagement of torque distribution clutches in a rear differential gear. Hence, backlash of the gears vanishes at once because of load of driving of the sub-driving wheels, and this causes rattling noise. Moreover, since rattling noise is continuously caused by the change in engine speed, noise/vibration (NV) performance and merchantability are reduced.
Unfortunately, when a deceleration state or a coasting state in which the vehicle moves only by inertia is changed to an acceleration state, rattling noise cannot be sufficiently reduced only by normal torque distribution control that transmits the driving torque to the sub-driving wheels via the rear differential gear. Hence, there is a demand to further reduce rattling noise. In addition, on a road surface having a low coefficient of friction, if the torque rise characteristic of the sub-driving wheels delays while the accelerator pedal is pressed during steering (turning), driving is performed only by the main driving wheels (front wheels) for a moment, and this may cause understeering during turning.
Accordingly, it is desirable to effectively reduce rattling noise caused in gears when a driving state of a four-wheel drive vehicle is changed from a deceleration state or a coasting state in which the vehicle moves only by inertia to an acceleration state, and to improve understeering to neutral steering when an accelerator pedal is pressed on a road surface having a low coefficient of friction.